666 SCIENCE PROGRESS 



with acids to form highly coloured salts without the necessary 

 formation of a chromophoric group — e.g. a quinonoid. Halo- 

 chromism has been chiefly studied by Baeyer and Villiger, 1 

 Kehrmann, 2 Kauffmann, 3 and others, and has generally been 

 connected with certain oxygen bodies. The oxygen in these 

 compounds possesses weakly basic properties, and in salt for- 

 mation is supposed to pass from the divalent to the tetravalent 

 condition. The class of compounds showing halochromism is 

 very varied and extensive, and includes in particular many 

 ketones and phenols and their ethers. In the case of triphenyl- 

 carbinol, which dissolves in concentrated sulphuric acid with a 

 deep yellow colour, Baeyer and Villiger ascribe metallic pro- 

 perties to the (C 6 H 5 ) 3 C radicle, so that consequently sulphuric 

 acid produces a carbonium-salt. 



It seems highly probable that halochromism is also due to 

 intra-molecular rearrangement, but the absence of sufficient 

 material leaves this a matter of hypothesis. 



Fluorescence 



Although the subject of fluorescence is, strictly speaking, 

 outside the scope of this article, yet it possesses a certain 

 interest when viewed simply as a variation of the property of 

 colour. This is particularly the case regarding the theory of 

 Hewitt. 4 It has been shown by Hewitt in a large number of 

 cases that fluorescent substances possess the ability to tauto- 

 merise, and from this he infers that fluorescence is produced 

 by the absorption of light of a certain wave-length by one 

 form, and the emission of the light in a different wave-length 

 by the other form. As an example may be quoted the members 

 of the fluorescein group — 



XXXT ? "°CC0 o " $ xccf 



C 6 H 4 < C 8 H 4 / \o C 6 H 4 / 



COOH \CO/ \COOH 



1 Baeyer and Villiger, Ber. 35, 1189, 1754, 3013 (1902) ; 36, 2774 (1903). 

 8 Kehrmann and Wentzel, Ber. 34, 3815 (1901). 

 3 Ber. 35, 1321 (1902) ; 36, 561 (1903). 

 * Zeit. f. physik. Chem. 34, 1 (1900). 



